The present invention relates to the verification of proper operation of equipment, and more particularly to a video error/distortion checker for evaluating whether equipment is operating within acceptable margins of error or is seriously malfunctioning.
A straight forward method of detecting errors in digital video equipment is to store in a buffer a copy of a repetitive signal and to compare the stored, or reference, signal to successive occurrences of the repetitive signal. An exact match between the reference signal and the received signal within specified limits indicates a properly functioning system. Various oscilloscopes, such as the Tektronix 2400 Series Digitizing Oscilloscopes manufactured by Tektronix, Inc. of Wilsonville, Oreg., United States of America, implement automatic pass/fail testing by comparing incoming signals against reference waveforms and alerting the operator in some manner if the incoming signal is out of limits. In two application areas such exact comparisons are not an adequate indicator of equipment quality. The first application is for PAL composite digital systems, and the second application is where non-lossless data compression is used.
In the first application a PAL composite digital signal is sometimes generated in a form commonly called "1135H" for its low cost, and then processed by a method called "sample rate conversion" to a form that is commonly called "4FSC", which is the required form for PAL composite digital signals. The sample rate conversion process introduces small uncertainties in the signal data, and the results of the introduced uncertainties vary over time. A constant signal in 1135H form processed to 4FSC form is no longer constant from video frame to video frame.
In the second application data compression is used to express an image or signal in a form that requires fewer data than an uncompressed signal. The compressed signal is ultimately decompressed, but the compression and decompression processing usually results in a signal that does not exactly match the original uncompressed signal.
What is desired is a video error/distortion checker that can distinguish between digital equipment operating within normal and acceptable error margins and digital equipment that is seriously malfunctioning for signals that are not exact replications of the original signal.